Microwave ovens use electromagnetic energy at frequencies that vibrate molecules within a food product to produce heat. The heat so generated warms or cooks the food. However, the food is not raised to a sufficiently high temperature to brown its surface to a crisp texture (and still keep the food edible).
To achieve these visual and tactile aesthetics a susceptor formed of a substrate having a lossy susceptor material thereon may be placed adjacent to the surface of the food. When exposed to microwave energy the material of the susceptor is heated to a temperature sufficient to cause the food's surface to brown and crisp.
The walls of a microwave oven impose boundary conditions that cause the distribution of electromagnetic field energy within the volume of the oven to vary. These variations in intensity and directionality of the electromagnetic field, particularly the electric field constituent of that field, create relatively hot and cold regions in the oven. These hot and cold regions cause the food to warm or to cook unevenly. If a microwave susceptor material is present the browning and crisping effect is similarly uneven.
To counter this uneven heating effect a turntable may be used to rotate a food product along a circular path within the oven. Each portion of the food is exposed to a more uniform level of electromagnetic energy. However, the averaging effect occurs along circumferential paths and not along radial paths. Thus, the use of the turntable still creates bands of uneven heating within the food.
This effect may be more fully understood from the diagrammatic illustrations of FIGS. 1A and 1B.
FIG. 1A is a plan view of the interior of a microwave oven showing five regions (H1 through H5) of relatively high electric field intensity (“hot regions”) and two regions C1 and C2 of relatively low electric field intensity (“cold regions”). A food product F having any arbitrary shape is disposed on a susceptor which, in turn, is placed on a turntable T. The susceptor is suggested by the dotted circle while the turntable is represented by the bold solid-line circle. Three representative locations on the surface of the food product F are illustrated by points J, K, and L. The points J, K, and L are respectively located at radial positions P1, P2 and P3 of the turntable T. As the turntable T rotates each point follows a circular path through the oven, as indicated by the circular dashed lines.
As may be appreciated from FIG. 1A, during one full revolution point J passes through a single region H1 of relatively high electric field intensity. During the same revolution the point K passes through a single smaller region H5 of relatively high electric field intensity, while the point L experiences three regions H2, H3 and H4 of relatively high electric field intensity. Rotation of the turntable through one complete revolution thus exposes each of the points J, K, and L to a different total amount of electromagnetic energy. The differences in energy exposure at each of the three points during one full rotation is illustrated by the plot of FIG. 1B.
Owing to the number of hot regions encountered and cold regions avoided, points J and L experience considerably more energy exposure than Point K. If the region of the food product in the vicinity of the path of point J is deemed fully cooked, then the region of the food product in the vicinity of the path of point L is likely to be overcooked or excessively browned (if a susceptor is present). On the other hand, the region of the food product in the vicinity of the path of point K is likely to be undercooked.
Since non-uniform cooking due to the presence of hot and cold regions is undesirable it has been found advantageous to employ a susceptor assembly formed by the combination of a field director structure with a susceptor. The field director structure includes one or more vanes, each having a conductive portion on a paperboard support. The field director structure mitigates the effects of regions of relatively high and low electric field intensity within a microwave oven by redirecting and relocating these regions so that food warms, cooks and browns more uniformly. Use of the field director structure alone (i.e., without a susceptor) has also been found advantageous.
When a susceptor assembly is placed in an “unloaded” microwave oven (i.e., an oven without a food product or other article being present) and the oven is energized deleterious problems of overheating of the susceptor, and/or overheating of the field director structure, and/or arcing have been observed.
By “overheating of the susceptor” (or similar terms) it is meant heating of the lossy susceptor material to the extent that the susceptor substrate burns.
“Overheating of the field director structure” (or similar terms) means heating of the paperboard support of the vanes to the extent that it burns. Such overheating may be caused by either the heat generated by a lossy susceptor material or by arcing.
“Arcing” (or similar terms) is an electrical discharge occurring when a high intensity electric field exceeds the breakdown threshold of air. Arcing typically occurs in the vicinity of the electrically conductive portions of the vanes, particularly along the edges, and especially at any sharp corners. Arcing may cause the paperboard support of the vanes to discolor, to char, or, in the extreme, to ignite and to burn.
Most common expedients to prevent arcing are impractical in microwave oven applications. These expedients are also not suitable for disposable packaging for convenience foods.
In view of the foregoing it is believed advantageous to provide a field director structure and a susceptor assembly incorporating the same that prevents the occurrence of arcing, the occurrence of overheating of the field director, and the occurrence of overheating of the susceptor.